Kepler Discovers Earth-Sized Mystery Planet

For scientists who hunt for planets in other solar systems, the "Holy Grail" is to find a rocky, earth-sized world that orbits its star at a distance where it's not too hot and not too cold for liquid water to form. Today in the journal Nature, a team led by Andrew Howard announced the discovery of Kepler-78b, a planet that meets two of the three requirements.

With a radius 1.2 times the size of Earth's its density, Kepler-78b looks a lot like our homeworld. The planet is probably made = of rock and iron, just like Earth. Located 400 light-years away in the Cygnus constellation, Kepler-78b is the first Earth-sized Kepler-78b exoplanet found to have a rocky composition. But Kepler-78b isn't a total jackpot. It has an 8.5-hour day and orbits close to its star, making it about 2000 degrees hotter than Earth, and therefore unlikely to support alien life.

The planet was discovered in the data from the Kepler space telescope, which watched for stars to pass in front of the stars they orbit. As Kepler-78b eclipsed its star, scientists used the dip in light levels to estimate the size of the planet. Next, ground-based telescopes on Earth measured the planet's mass by monitoring how much the planet tugs on its star. Together, the size and mass revealed the planet's density.

Typically, it is difficult to measure the mass of planets that Kepler finds because they're hundreds of light-years away, making it hard for ground-based telescopes to spot the subtle wobble of the star. But since Kepler-78b orbits dangerously close to its star, the planet exerts a greater gravitational pull on the star that it would if it were as far as Earth is from our sun.

The 8.5-hour orbital period also places Kepler-78b into a new class of poorly understood planets. According to some accepted theories, Kepler-78b is so close to its star that it shouldn't exist. It couldn't have formed in its current location, because at the time of its formation, the star was larger and would have swallowed the planet. But it couldn't have migrated there either, says Harvard-Smithsonian astronomer Dimitar Sasselov, an author on one of the new papers.

"It couldn't have formed in place because you can't form a planet inside a star," he said in a statement. "It couldn't have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma."

The next step for exoplanet scientists is to keep searching for a planet that is not only rocky and earth-sized, but lives in a more hospitable zone around its star. In a Nature commentary, astronomer Drake Deming from the University of Maryland writes that the TESS telescope, which is scheduled to launch in 2017 and will search for earth-like planets closer to home, could find such a world. And the fact that its discoveries should be closer to Earth will make it easier for ground-based telescopes to measure the mass of planets that are further from their host stars.

"The existence of Kepler-78b shows that, at the very least, extrasolar planets of Earth-like composition are not rare," Deming says.

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